A wireless local area network (WLAN) in infrastructure basic service set (BSS) mode has an Access Point (AP) for the BSS and one or more stations (STAs) associated with the AP. The AP typically has access to or interfaces with a distribution system (DS) or another type of wired/wireless network that carries traffic in and out of the BSS. Traffic to the STAs that originates from outside the BSS arrives through the AP and is delivered to the STAs. Traffic originating from the STAs to destinations outside the BSS is sent to the AP to be delivered to the respective destinations. Traffic between STAs within the BSS may also be sent through the AP where the source STA sends traffic to the AP and the AP delivers the traffic to the destination STA. Such traffic between STAs within a BSS is really peer-to-peer traffic. Such peer-to-peer traffic may also be sent directly between the source and destination STAs with direct link setup (DLS) using an 802.11e DLS or an 802.11z tunneled DLS (TDLS). A WLAN in Independent BSS mode has no AP, and STAs communicate directly with each other.
Enhanced distributed channel access (EDCA) is an extension of the basic Distributed Coordination Function (DCF) introduced in 802.11 to support prioritized Quality of Service (QoS). The operation of EDCA in 802.11n is shown in FIG. 1.
The point coordination function (PCF) uses contention-free channel access, and includes the following properties: supports time-bounded services; polling by the AP; the AP sends a polling message after waiting for a PCF interframe space (PIFS); if a client has nothing to transmit, it returns a null data frame; since the PIFS is smaller than a DCF interframe space (DIFS), it can lock out all asynchronous traffic; it is deterministic and fair; and it is efficient for both low duty-cycle and heavy/bursty traffic.
The hybrid coordination function (HCF) controlled channel access (HCCA) is an enhancement of PCF. The AP can poll a STA during both a contention period (CP) and a contention-free period (CFP), and may transmit multiple frames under one poll.
The traffic indicator message (TIM)-based power saving mechanism may be used in 802.11. Basic power management modes are defined and include Awake and Doze states. The AP is aware of the current power saving modes used by STAs it is addressing and buffers the traffic status for STAs that are in a sleep, or Doze state. The AP notifies corresponding STAs using the TIM/delivery traffic indication messages (DTIM) in beacon frames. The STA, which is addressed by the AP, may achieve power savings by entering into the Doze state, and waking up to listen for beacons to receive the TIM to check if the AP has buffered traffic for it to receive. The STA may send a power saving (PS)-Poll control frame to retrieve buffered frames from the AP. The STAs may use a random back-off algorithm before transmitting the PS-Poll frames if and when multiple STAs have buffered frames waiting for reception from the AP.
An example of the TIM and DTIM operation is shown in FIG. 2. The TIM is indicated using an association identifier (AID) bitmap or a partial virtual bitmap. The STAs that currently have buffered bufferable units (BUs) within the AP are identified in a TIM, which is included as an element in all beacon frames generated by the AP. A STA determines that a BU is buffered for it by receiving and interpreting the TIM.
In a BSS operating under the DCF or during the CP of a BSS using the PCF, upon determining that a BU is currently buffered in the AP, a STA operating in the PS mode transmits a short PS-Poll frame to the AP. The AP responds with the corresponding buffered BU immediately, or acknowledges the PS-Poll and responds with the corresponding BU at a later time. If the TIM indicating the buffered BU is sent during a CFP, a CF-Pollable STA operating in the PS mode does not send a PS-Poll frame, but remains active until the buffered BU is received or the CFP ends.
At every beacon interval, the AP assembles the partial virtual bitmap containing the buffer status per destination for STAs in the PS mode and sends the bitmap in the TIM field of the beacon frame. At every beacon interval, the automatic power-save delivery (APSD)-capable AP assembles the partial virtual bitmap containing the buffer status of non-delivery-enabled access categories (ACs) (if there exist at least one non-delivery-enabled AC) per destination for STAs in the PS mode and sends the bitmap in the TIM field of the beacon frame. When all ACs are delivery-enabled, the APSD-capable AP assembles the partial virtual bitmap containing the buffer status for all ACs per destination. If flexible multicast service (FMS) is enabled, the AP includes the FMS descriptor element in every beacon frame. The FMS descriptor element indicates all FMS group addressed frames that the AP buffers.
The maximum length of an information element in the current 802.11 standards is 256 bytes, which is determined by the one-byte length field in the element format. Consequently, this maximum Information Element (IE) size limits the number of STAs that can be supported in the TIM IE, as the TIM uses the bitmap to signal the STAs with buffered downlink (DL) BUs by mapping the STA's AIDs to the bits in the bitmap. In addition to the bitmap field, the TIM element also contains other information fields, for example, DTIM Count, DTIM Period, and Bitmap Control. Therefore, the maximum size of the bitmap field in the TIM element is further limited to 251 bytes.
For the current maximum of 2007 AIDs, the full bitmap needs 2007 bits (251 bytes), which is the maximum size of the bitmap field in the TIM. Therefore, the current TIM with its bitmap structure cannot support more than 2007 STAs. The length of the TIM element increases as the number of supported STAs increases, based on the current TIM element structure as specified in the 802.11 standards. For example, with a maximum of 2007 STAs, the worst case bitmap in the TIM element is 251 bytes. If the maximum number of STAs is increased to a larger number, for example, 6000, then the worst case bitmap would be 6000/8=750 bytes. Such a large size TIM increases the overhead of the TIM/beacon transmission and likely takes it out of the acceptable level, particularly in systems where the channel bandwidth, for example, 1 MHz, 2 MHz, up to 8 MHz, is smaller than other systems.
The following provides an example of analyzing the TIM/beacon overhead in an 802.11ah system, by assuming that a typical beacon has a size of 230 bytes and a transmission rate of 100 Kbps. If a typical beacon frame contains a 30-byte TIM element out of a 230-byte beacon frame, it implies that there are 200 bytes of non-TIM content in a beacon frame.
Considering that a typical bitmap may be smaller than the worst case, it is assumed that the typical size of a TIM bitmap is one-third of the worst case, i.e., 250 bytes for 6000 STAs, which results in a 255-byte TIM element. The beacon frame would be 200+255=455 bytes, counting 200 bytes of non-TIM content. If the beacon is transmitted at the rate of 100 Kbps, the 455-byte beacon frame will take at least 455×8/100=36.4 ms to transmit. Since the beacon interval is typically set up to be 100 ms, beacon frame overhead is 36.4%, which does not include the time used for channel access and inter-frame spacing. For the worst case scenario in which the TIM size is 755 bytes, the beacon frame would be 200+755=955 bytes, corresponding to a 76.40 ms transmission time, or 76.4% of a 100-ms beacon interval.
The Power-Save Multi-Poll (PSMP) mechanism is introduced in 802.11n and has the following features. It may use a single PSMP frame to schedule multiple STAs instead of the direct QoS (+) CF-Poll used in HCCA. The scheduling is more efficient under the scenario where the STAs periodically transmit a small amount of data. It may reduce power consumption by providing an uplink (UL) and a DL schedule at the start of the PSMP phase, so that each STA may shut down their receivers until needed in the DL phase and transmit when scheduled during the UL phase without performing clear channel assessment (CCA). An example of PSMP operation of three STAs is shown in FIG. 3.